The present invention relates to improvements in or relating to linear low density polyethylenes (LLDPEs), in particular to the use of LLDPEs for injection moulding and to products obtainable thereby.
LLDPEs are widely used in the manufacture of packaging products, which are typically produced by moulding techniques, especially injection moulding. LLDPE materials used for these purposes are typically produced using conventional Ziegler-Natta catalysts.
In cases where an injection moulded LLDPE product is to be used in critical applications, e.g. in the packaging of food or medical products, particularly as closure means (e.g. lids) for food containers, it is essential that this should not contaminate the packaged product. For food packaging applications, an indication of the degree of contamination may be obtained from tests which determine the level of migration of the polymer material, e.g. when immersed in a fatty food simulant such as olive oil. In the case of LLDPEs prepared using Ziegler-Natta catalysts the levels of migration have been found to be too high to permit their use in the production of injection moulded packaging materials for food and medical products, especially fatty foods.
Surprisingly, we have now found that by using LLDPEs produced using a single site catalyst, in particular those produced using a metallocene catalyst (m-LLDPEs), it is possible to produce moulded products (e.g. injection moulded products) having acceptable migration levels for use in packaging food and medical products, especially for use in packaging foods having a high fat content, such as cheese, mayonnaise, ketchup, butter, etc.
Thus viewed from one aspect the invention provides the use of an LLDPE produced using a single site catalyst, in particular an LLDPE produced using a metallocene (m-LLDPE), in injection moulding of food packaging material, especially closures for food containers.
Particularly preferred for use in the invention are LLDPEs having a relatively narrow molecular weight distribution or MWD (i.e. the ratio of the weight average molecular weight to the number average molecular weight), e.g. those having a MWD ranging from 2 to 60, preferably from 3 to 10, more preferably from 3 to 6.
Viewed from a further aspect the invention provides an injection moulded article, e.g. an injection moulded closure, particularly a closure for a food container, formed from an LLDPE produced using a single site catalyst, preferably from a metallocene LLDPE, said LLDPE preferably having a MWD in the range of from 2 to 60, preferably from 3 to 10, more preferably from 3 to 6.
LLDPE materials which have been found to be particularly suitable for use in the production of injection moulded packaging materials for food and medical products are those having migration levels of less than 40 mg/dm2, preferably less than 10 mg/dm2, e.g. less than 5 mg/dm2.
Viewed from a further aspect the invention thus provides an LLDPE, e.g. a metallocene LLDPE, suitable for use in injection moulding having a migration level of less than 40 mg/dm2, preferably less than 10 mg/dm2, e.g. less than 5 mg/dm2.
Viewed from a still further aspect the invention provides the use of an LLDPE, e.g. a metallocene LLDPE, in forming an article, preferably a closure for a container, having a migration level of less than 40 mg/dm2, preferably less than 10 mg/dm2, e.g. less than 5 mg/dm2.
By polyethylene is meant a polymer the majority by weight of which derives from ethylene monomer units. A minor proportion, e.g. not more than 20% by weight, more preferably not more than 15% by weight, of the polymer may derive from other monomers copolymerisable with ethylene. Suitable comonomers include those selected from C3-20 mono or multiple unsaturated monomers, in particular C3-10 xcex1-olefins, e.g. propene, but-1-ene, pent-1-ene, 3-methyl-but-1-ene, 4-methyl-pent-1-ene, hex-1-ene, 3,4-dimethyl-but-1-ene, hept-1-ene, 3-methyl-hex-1-ene, etc. Preferably the monomers will be selected from propene, but-1-ene, hex-1-ene and oct-1-ene. As used herein, ethylene copolymer is intended to encompass a polyethylene deriving from ethylene and one or more monomers copolymerisable with ethylene.
The polyethylene may also contain minor amounts, e.g. not more than 10% by weight, preferably not more than 5% by weight, of other polymers, e.g. other polyolefins such as polypropylenes. Conventional additives such as antioxidants, UV-stabilizers, colours, fillers, etc., generally in amounts of up to 10% by weight, e.g. up to 5% by weight, may also be present.
By LLDPE is meant a polyethylene having a density of 890 to 940 kg/m3, preferably 915 to 930 kg/m3, especially 917 to 926 kg/m3, and a crystallinity of 20 to 60%, preferably 30 to 50%, especially 40 to 50%.
LLDPEs useful in the invention include mono-modal, bi-modal and multi-modal polymers. Mono-modal polymers, which are typically characterised by a narrow molecular weight distribution, may be advantageous. Typically a mono-modal polymer having a narrow MWD will be produced in a single polymerization stage under a single set of processing conditions (temperature, pressure, etc.) using a single monomer and a single polymerization catalyst.
Bi-modal and multi-modal LLDPEs useful in the invention may be produced by blending two or more mono-modal polyethylenes having different MWDs. Alternatively and preferably, they may be produced by polymerization under conditions which create a bi-modal or multi-modal polymer, e.g. using two or more single site catalysts and/or using a catalyst system or mixture with two or more different catalytic sites, or using two or more polymerization stages in which the reactants are subjected to different reaction conditions (e.g. different temperatures, pressures, polymerization media, hydrogen partial pressures, etc.) (see EP-A-778289).